Conventional converter circuits have a converter unit with a multiplicity of drivable power semiconductor switches, which are connected in a known manner in order to switch at least two switching voltage levels. An LCL filter is connected to each phase connection of the converter unit. A capacitive energy store is also connected to the converter unit and is normally formed by one or more capacitors. An apparatus is provided for operating the converter circuit, which has a control device for producing a hysteresis power value, a hysteresis wattless component value and a selected flux sector, which control device is connected to the drivable power semiconductor switches via a drive circuit in order to form a drive signal from the hysteresis power value, the hysteresis wattless component value and the selected flux sector. The power semiconductor switches are therefore driven by means of the drive signal.
A converter circuit as mentioned above is subject to the problem that the LCL filters can cause permanent distortion, that is to say undesirable oscillations, in the filter output currents and filter output voltages as a result of resonant oscillations of the LCL filters, as shown in the normal waveform of filter output currents shown in FIG. 3. In an electrical AC voltage power supply system, which is typically connected to the filter outputs, or when an electrical load is connected to the filter outputs, such distortion can lead to damage or even to destruction, and is therefore very highly undesirable.